Carbon dioxide volume estimated from seismic data after six years of injection in the oil field of Buracica, Bahia

摘要

Carbon dioxide has been injected since 1991 in the oil field of Buracica in the Reconcavo Basin in Brazil for EOR purposes. The CO_2 gas is injected into the upper oil reservoirs, a 13 m-thick sandstone layer, at a depth of about 550 m. The reservoir is included in a tilted block dipping at an angle of 5 to 6° toward the south-east. A 3D seismic survey was carried out six years after the beginning of CO_2 injection. Sensitivity studies concluded that the gas-invaded and the oil-filled parts of the reservoir show only a weak contrast between their mechanical properties so that their interface might not appear in the seismic sections. Directional dip filtering of the seismic data underlines horizontal events crossing the dipping layer interfaces. Some of them can be interpreted as the gas/oil contact. A careful picking and mapping of these events reveal two accumulations of carbon dioxide on each side of a system of N-S faults, with slightly different gas/oil contact levels. Estimation of the gas volume and of the density leads to a rough estimate of the mass of CO_2 in place, indicating that about one third of the CO_2 injected was stored in the reservoir.